Recent advancements in computer vision have driven the development of speech-driven 2D digital human lip-sync animation technology, which is now widely applied in fields such as animation, virtual idols, and online education. However, despite significant investments in improving relevant technical metrics, one crucial question remains underexplored: whether these optimizations truly enhance user cognition, as humans are the ultimate audience. To address this, we conducted a series of experiments by presenting digital human videos with varying technical quality levels and recording participants’ performance in information acquisition and cognitive tasks. Our goal was to assess whether higher technical metrics lead to better user outcomes. The results indicate that while higher technical metrics do improve information retention and visual clarity, they do not necessarily enhance subjective experience or learning outcomes, exhibiting a “plateau effect.” This finding highlights the need to integrate both objective and subjective evaluation criteria when optimizing speech-driven lip synchronization technology, particularly for educational contexts, and offers valuable guidance for the future development and optimization of this technology.